PROJECT SUMMARY HIV-1 continues to spread at an alarming rate, with an estimated 4 million new infections occurring each of the last several years. There are currently no HIV-1 vaccine candidates that demonstrate efficacy against the diverse HIV-1 strains that are circulating globally. Neutralizing antibodies have been shown to block HIV-1 infection in experimental model systems. However, these results were obtained under ideal conditions, where passively administered antibodies that were known to readily neutralize the virus being tested were used; most HIV-1 strains would not be neutralized by these same antibodies. At present, it is unclear what types of antibodies would be most effective in blocking the strains of HIV-1 that are circulating in highly affected populations. Antibodies are passively acquired in the setting of mother-to-child HIV-1 transmission, and our recent studies suggest that these antibodies may select for transmission of escape variants to the infant. Thus, the exposure of infants to HIV-1 from their mother provides a setting in which to examine the role of neutralizing antibodies in HIV-1 transmission, and to characterize the specificity of antibody responses that may be protective. We hypothesize that there are neutralizing antibodies that contribute to protection of the infant from HIV-1 infection. Here, we propose to test this hypothesis using banked samples collected from a clinical trial of breastfeeding transmission of HIV-1 that included 425 mother-infant pairs from Nairobi, Kenya. In this cohort, infant infection status was monitored at regular intervals, so that the timing of infection is well defined; In addition a variety of clinical and virological data is available. Using samples from this cohort, maternal neutralizing antibody breadth and potency will be examined against a panel of HIV-1 variants isolated from early in infection. Passively transferred antibody profiles will be similarly evaluated in a subset of infants. The goal of these aims will be to identify the neutralizing antibody responses that correlate with a reduced risk of HIV-1 transmission, and specifically, whether potency, breadth or specificity for particular viral strains is the best predictor of transmission risk. In addition, we propose to examine the molecular basis for escape from neutralizing antibodies in cases where transmission has occurred. These studies will help define critical epitopes on the HIV-1 envelope protein that contribute to neutralization escape during HIV-1 transmission. Together, these studies will test the hypothesis that broad and/or potent neutralizing antibody responses can help protect against HIV-1 infection, and they may provide unique insights into the specificity of antibodies that are capable of blocking HIV-1 acquisition. Testing this hypothesis is of critical importance to future vaccine design, because it will provide data to support or refute the importance of eliciting neutralizing antibodies with a vaccine immunogen. PROJECT NARRATIVE A major goal of HIV vaccine research is to find a means to elicit broad and potent neutralizing antibodies. However, there is no direct evidence that such antibodies actually protect humans from HIV-1 infection. Here, we propose to test the hypothesis that broad and potent neutralizing antibodies protect infants of HIV- 1 positive mothers from infection.